Fix cascading settings attr writer behavior

If set value matches the cascaded value, do not save locally.
This fixes an issue where a simple settings form save by a subgroup
would cause a cascaded value to be saved at the subgroup level,
defeating future cascading lookups. Also, when setting a lock
attribute and the local setting is `nil`, copy the value locally to
ensure ancestors can't change the intended value for subgroups later.

Changelog: fixed

app/models/concerns/cascading_namespace_setting_attribute.rb

@@ -55,6 +55,7 @@ module CascadingNamespaceSettingAttribute
         # public methods
         define_attr_reader(attribute)
         define_attr_writer(attribute)
+        define_lock_attr_writer(attribute)
         define_lock_methods(attribute)
         alias_boolean(attribute)
 
@@ -97,7 +98,17 @@ module CascadingNamespaceSettingAttribute
 
     def define_attr_writer(attribute)
       define_method("#{attribute}=") do |value|
+        return value if value == cascaded_ancestor_value(attribute)
+
         clear_memoization(attribute)
+        super(value)
+      end
+    end
+
+    def define_lock_attr_writer(attribute)
+      define_method("lock_#{attribute}=") do |value|
+        attr_value = public_send(attribute) # rubocop:disable GitlabSecurity/PublicSend
+        write_attribute(attribute, attr_value) if self[attribute].nil?
 
         super(value)
       end

changelogs/unreleased/dblessing_cascading_override.yml

+---
+title: Fix cascading settings attr writer behavior
+merge_request: 59910
+author:
+type: fixed

spec/models/concerns/cascading_namespace_setting_attribute_spec.rb

@@ -142,7 +142,7 @@ RSpec.describe NamespaceSetting, 'CascadingNamespaceSettingAttribute' do
       end
 
       it 'does not allow the local value to be saved' do
-        subgroup_settings.delayed_project_removal = nil
+        subgroup_settings.delayed_project_removal = false
 
         expect { subgroup_settings.save! }
           .to raise_error(ActiveRecord::RecordInvalid, /Delayed project removal cannot be changed because it is locked by an ancestor/)
@@ -164,6 +164,19 @@ RSpec.describe NamespaceSetting, 'CascadingNamespaceSettingAttribute' do
     end
   end
 
+  describe '#delayed_project_removal=' do
+    before do
+      subgroup_settings.update!(delayed_project_removal: nil)
+      group_settings.update!(delayed_project_removal: true)
+    end
+
+    it 'does not save the value locally when it matches the cascaded value' do
+      subgroup_settings.update!(delayed_project_removal: true)
+
+      expect(subgroup_settings.read_attribute(:delayed_project_removal)).to eq(nil)
+    end
+  end
+
   describe '#delayed_project_removal_locked?' do
     shared_examples 'not locked' do
       it 'is not locked by an ancestor' do
@@ -277,6 +290,13 @@ RSpec.describe NamespaceSetting, 'CascadingNamespaceSettingAttribute' do
         expect { subgroup_settings.save! }
           .to raise_error(ActiveRecord::RecordInvalid, /Delayed project removal cannot be nil when locking the attribute/)
       end
+
+      it 'copies the cascaded value when locking the attribute if the local value is nil', :aggregate_failures do
+        subgroup_settings.delayed_project_removal = nil
+        subgroup_settings.lock_delayed_project_removal = true
+
+        expect(subgroup_settings.read_attribute(:delayed_project_removal)).to eq(false)
+      end
     end
 
     context 'when application settings locks the attribute' do